Biaxially stretched polyester films, such as a biaxially stretched polyethylene terephthalate film, have excellent strength, heat resistance, dimensional stability, chemical resistance, aroma retaining properties, and the like, and therefore are useful as materials for various packages. Therefore, packaging bags, such as flexible pouches, formed by heat-sealing such biaxially stretched polyester films with each other are expected to be produced. However, films having such orientation have poor heat sealing properties. In this regard, for example, Patent Literature 1 discloses a method of imparting heat sealing properties to a biaxially stretched polyester film by irradiating the surface of the film with short pulses of electromagnetic waves and modifying the surface.